Wastewater collection and treatment: an overview

Every flush, shower and load of laundry sends water into a sewer. Together with wastewater from businesses and industries, and in many older cities rain and snowmelt as well, it flows to a treatment plant that must remove enough organic matter, solids, nutrients and pathogens to protect the river or lake that receives the effluent. Wastewater treatment is the reason rivers that were open sewers a century ago now support fish, rowing clubs and drinking water intakes downstream. This lesson describes wastewater sources and characteristics, sewer systems and their problems, the stages of treatment from screens to disinfection, nutrient removal, biosolids, and the regulations that set effluent quality.

The Sioux Falls wastewater plant in Willowbend's examples treats an average of 30,000 m³/d from homes, businesses and a large food processing plant. (The real plant is larger; the numbers here are simplified.) Its effluent must meet a National Pollutant Discharge Elimination System (NPDES) permit that includes the federal secondary treatment standards and additional limits on ammonia and phosphorus to protect the Big Sioux River.

Where wastewater comes from

Domestic wastewater comes from toilets, sinks, showers, dishwashers and laundry. Typical residential water use in North America is roughly 200 to 350 liters per person per day, and most of it returns as wastewater. Commercial and institutional users add more, and industries such as meat packing, dairies, breweries and pulp mills can dominate a city's load. Many municipalities require industries to pretreat their waste under a sewer-use bylaw or pretreatment program so that it does not upset the plant or pass through it untreated. Groundwater leaking into cracked pipes (infiltration) and rainwater entering through roof leaders, foundation drains and manhole lids (inflow) add clear water that does not need treatment but takes up capacity; together they are called inflow and infiltration, or I/I.

Engineers often express an industrial load as a population equivalent, the number of people who would produce the same BOD load. A meat-packing plant discharging 2,400 kg of BOD a day has a population equivalent of about 30,000 people at 80 g per person per day.

Sewers

Sanitary sewers carry only wastewater; separate storm sewers carry rain and snowmelt straight to rivers. Older parts of many cities, including some Canadian and American downtowns built before about 1950, have combined sewers that carry both in one pipe. In dry weather all the flow goes to the treatment plant, but in heavy rain the flow exceeds the pipes' and plant's capacity and the excess overflows, untreated, to the river through combined sewer overflows (CSOs). CSOs are a major source of bacteria and floatable debris in urban rivers, and many cities are spending billions on separation, storage tunnels and treatment to control them. Sanitary sewers can overflow too, when I/I overwhelms them.

Most sewers flow by gravity, sloped so that the wastewater moves fast enough, typically at least 0.6 m/s, to keep solids from settling. Where the land is flat or the sewer must cross a rise, lift stations pump the flow to a higher point. The flat prairie landscape means that cities such as Brandon, Winnipeg, Fargo and Sioux Falls need many lift stations. Hydrogen sulfide generated in long, slow sewers and force mains causes odors and corrodes concrete, so sewer design pays attention to keeping wastewater fresh.

Stages of treatment

Preliminary treatment protects the equipment downstream. Screens remove rags, plastics and debris, and grit chambers let sand and gravel settle while lighter organic solids stay in suspension. Flow is measured here, and many plants add equalization basins to even out daily peaks.

Primary treatment is plain sedimentation. Primary clarifiers hold the wastewater for about two hours, during which roughly 50 to 70 percent of the suspended solids and 25 to 40 percent of the BOD settle out as primary sludge, while grease and scum float and are skimmed off. Primary treatment alone is not enough for most receiving waters; a few coastal cities long relied on it, but nearly all now provide more.

Secondary treatment uses microorganisms to consume the dissolved and colloidal organic matter that primary treatment cannot remove. The bacteria use the organic matter as food, converting part of it to carbon dioxide and water for energy and part of it into new cells, which are then settled out. The most common secondary process is activated sludge, in which wastewater is mixed with a suspension of microorganisms, the mixed liquor, in aerated tanks for several hours. The mixture flows to secondary clarifiers where the biological floc settles; most of it is returned to the aeration tanks to keep the microorganism population high, and the excess is wasted as waste activated sludge. Other secondary processes include trickling filters and rotating biological contactors, where the microorganisms grow on fixed media, and membrane bioreactors, which replace secondary clarifiers with membranes.

Small communities on the prairies very often use lagoons, large earthen ponds in which bacteria and algae treat wastewater slowly over months. Facultative lagoons have an aerobic upper layer, kept supplied with oxygen by algae and wind, above an anaerobic bottom layer. Because lagoons freeze in winter, many prairie systems store wastewater through the winter and discharge treated effluent only in spring and fall when the receiving stream can accept it. Lagoons are cheap to build and operate but need a lot of land and struggle to meet ammonia limits in cold weather.

Designing the activated sludge process

Two parameters summarize an activated sludge design. The food-to-microorganism ratio (F/M) compares the daily BOD load with the mass of microorganisms in the aeration tank, measured as mixed liquor volatile suspended solids (MLVSS):

Here S₀ is the influent BOD, V the aeration tank volume and X the MLVSS concentration. Conventional plants run at F/M of about 0.2 to 0.4 per day. The solids retention time (SRT), or sludge age, is the average time microorganisms stay in the system, equal to the mass of solids in the aeration tank divided by the mass wasted per day. A long SRT, typically 8 to 20 days in cold climates, is needed for the slow-growing nitrifying bacteria that convert ammonia to nitrate.

Removal efficiencies and loads

Removal efficiency is the fraction of a pollutant removed, E = (C_in − C_out)/C_in. Following loads through each stage of a plant is a direct application of the mass balance from lesson 3.

# BOD and TSS through the plant (concentrations in mg/L, flow in m3/d)
Q = 30000
raw = {"BOD5": 220.0, "TSS": 240.0}
primary_removal = {"BOD5": 0.33, "TSS": 0.60}
final = {"BOD5": 12.0, "TSS": 15.0}          # secondary effluent (CBOD for BOD5)
for p in raw:
    pe = raw[p] * (1 - primary_removal[p])
    print(f"{p}: raw {raw[p]:.0f} -> primary {pe:.0f} -> final {final[p]:.0f} mg/L")
    print(f"      loads: {raw[p] * Q / 1000:,.0f} -> {pe * Q / 1000:,.0f} -> {final[p] * Q / 1000:,.0f} kg/d,"
          f" overall removal {100 * (1 - final[p] / raw[p]):.1f}%")
F_M, X = 0.30, 2500                            # 1/d, mg/L MLVSS
S0 = raw["BOD5"] * (1 - primary_removal["BOD5"])
V = Q * S0 / (F_M * X)
print(f"Aeration volume = {V:,.0f} m3, HRT = {24 * V / Q:.1f} h")

BOD5: raw 220 -> primary 147 -> final 12 mg/L
      loads: 6,600 -> 4,422 -> 360 kg/d, overall removal 94.5%
TSS: raw 240 -> primary 96 -> final 15 mg/L
      loads: 7,200 -> 2,880 -> 450 kg/d, overall removal 93.8%
Aeration volume = 5,896 m3, HRT = 4.7 h

The plant removes about 95 percent of the BOD and 94 percent of the suspended solids, cutting the BOD load to the river from 6,600 kg/d to 360 kg/d. Primary treatment does a large share of the solids removal cheaply, while secondary treatment does most of the BOD removal.

Nutrient removal

Conventional secondary treatment removes only a small part of the nitrogen and phosphorus. Where receiving waters are sensitive, plants add nutrient removal. Phosphorus can be removed chemically by adding iron or aluminum salts that precipitate it into the sludge, or biologically by creating anaerobic zones that encourage bacteria able to take up extra phosphorus, a process called enhanced biological phosphorus removal. Nitrogen is removed in two steps: nitrification converts ammonia to nitrate under aerobic conditions, and denitrification converts nitrate to nitrogen gas in anoxic zones where bacteria use nitrate instead of oxygen. Biological nutrient removal plants arrange anaerobic, anoxic and aerobic zones in sequence to achieve both. Achieving 1 mg/L of total phosphorus is routine with chemical addition; reaching 0.1 mg/L or less requires tertiary filtration or membranes.

Disinfection

Effluent disinfection reduces bacteria and viruses before discharge to waters used for swimming or downstream water supply. Ultraviolet light is now the most common choice because it adds no chemicals. Where chlorine is used, the effluent must usually be dechlorinated, often with sodium bisulphite, because chlorine residuals are toxic to fish; this is why effluent limits for total residual chlorine are so low. Some northern plants disinfect only in the recreation season.

Biosolids

Treatment removes pollutants from water but concentrates them in sludge, which can make up a third or more of a plant's operating cost. Sludge is thickened and then stabilized to reduce pathogens, odors and its tendency to rot. Anaerobic digestion, in which bacteria in heated, sealed tanks convert organic matter to biogas that is about 60 to 65 percent methane, is the most common stabilization process at medium and large plants; the biogas heats the digesters and buildings and can generate electricity or be upgraded to renewable natural gas. The stabilized sludge, called biosolids once it meets quality standards, is dewatered and applied to farmland as a fertilizer and soil conditioner, composted, landfilled or incinerated. Concerns about PFAS, microplastics and other contaminants in biosolids are prompting new testing requirements in several jurisdictions.

Effluent regulations

Under the Clean Water Act, every point source discharging to waters of the United States needs an NPDES permit. The EPA's secondary treatment regulation sets the minimum for publicly owned treatment works: a 30-day average BOD₅ of 30 mg/L and 7-day average of 45 mg/L (CBOD₅ of 25 and 40 mg/L may be substituted), the same 30 and 45 mg/L for suspended solids, at least 85 percent removal of each, and pH between 6.0 and 9.0. Where those limits would not protect the receiving water, permits add water-quality-based limits, such as ammonia, phosphorus and bacteria limits, derived from the state's water quality standards. Pretreatment programs control industrial discharges to municipal sewers.